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Q                                       The International Union of Geological
                               quartzolite                                       Sciences (IUGS) commission aimed to
                                                                                 simplify the nomenclature, eliminate
                                        90    90
                                                                                 synonymic terms (e.g., adamellite =
                                                                                 quartz monzonite), and come up with
                                        quartz-rich
                                         granitioids                             fields and corresponding names that are
                                                                                 consistent with general usage. The IUGS
                                  60               60                            method relies on two parameters, the
                               alkali feldspar granite syeno- granite monzo-  granodiorite  tonalite  quartz monzodiorite,  modal percentage of quartz or feld-
                                                                                 spathoid minerals (horizontal lines), and
                                                          quartz-monzogabbro
           quartz alkali feldspar syenite  granite  granite   quartz diorite,    the ratio of alkali feldspar to plagioclase
                                                              quartz gabbro,
                                                                                 (lines radiating from the quartz and feld-
                           20    10     35      65    90  20  quartz anorthosite  spathoid apices). These values are easy to
             alkali feldspar syenite  quartz  quartz           monzodiorite,
                                 syenite  monzonite            monzogabbro       estimate in thin section, although less so
                          5                                 5
                                         monzonite
                                syenite
                       A       foid-bearing  foid-bearing     P                  in the field, where the plagioclase/alkali
                 foid-bearing  10  syenite  monzonite       10  diorite,         feldspar distinction can be subtle. In con-
                                                                gabbro,
            alkali feldspar syenite  10     50          90      anorthosite      trast, the modal proportions of mafic min-
                                                   foid diorite, foid gabbro  foid-bearing monzodiorite  diagnostic of a given rock unit, although
                                 foid monzosyenite foid monzodiorite,            erals and their sum (color index) are gen-
                                            foid monzogabbro  foid-bearing diorite,  erally easy to estimate in the field and
                                                             foid-bearing gabbro,
                                                             foid-bearing anorthosite
                              foid syenite
                                                                                 these are not used in the IUGS system.
                                                                                 way to assign a standardized name once
                                  60               60        foid-bearing monzogabbro  The IUGS diagram provides a convenient
                                                                                 the mode has been estimated, and this is
                                         foidolite                               both a strength and a significant problem.
                                                                                  As an example of the problems that this
                                        90    90                                 kind of classification can cause, consider
                                                                                 the data in Figure 3, taken from Bateman
                                           F                                     (1992). Modal data from the well-known
                                                                                 Cathedral Peak Granodiorite of Yosemite
          Figure 2. The  International Union  of Geological Sciences classification  double triangle. Field   National Park in California, USA (Fig. 1),
          boundaries are arbitrary and not designed to follow petrologic processes, so closely related rocks
          can scatter across several fields. The plethora of names and positions of boundary lines are diffi-  are split rather evenly between the granite
          cult to remember, particularly because they mean little in terms of process. They also serve to   and granodiorite fields. Thus, any random
          carve up related rocks (e.g., one map unit) into several different rock types.
                                                                                 hand sample or outcrop of the Cathedral
                                                                                 Peak pluton might be a granite or a grano-
                                                                                 diorite or straddle the arbitrary boundary
                                                                                 between them. This unit was designated
                                                                                 a granodiorite because one or the other
                                                                                 name had to be chosen, even though the
                                                                                 pluton includes both. The nearby El
                                                                                 Capitan Granite (Fig. 1) is also more-or-
                                                                                 less evenly divided between the granite
                                                                                 and granodiorite fields (with several
                                                                                 points in the tonalite field), but it is offi-
                                                                                 cially a granite. Rocks of the Kuna Crest
                                                                                 suite, a set of medium-grained rocks with
                                                                                 high color index, scatter over the grano-
                                                                                 diorite, tonalite, quartz monzodiorite/
                                                                                 quartz monzogabbro, and quartz diorite/
                                                                                 quartz gabbro fields. This sort of artificial


                                                                                 Figure 3. Modal data for plutonic units from
                                                                                 Yosemite National Park, California, USA, from
                                                                                 Bateman (1992). Many of the main units scat-
                                                                                 ter evenly across the granite (monzogranite)
                                                                                 and granodiorite fields, and petrologic vari-
                                                                                 ability in the granodiorite of Kuna Crest
                                                                                 crosses the common point of four fields,
                                                                                 meaning that the rock could be called any of
                                                                                 the four (really, six; see text). Q—quartz;
                                                                                 A—alkali feldspar; P—plagioclase.

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